Properly handle file paths with sites/:id

[mjangda]

Those need to be stripped out as well since the files paths are stored in postmeta without it.

Changelog Description

Private Files: Handle /sites/ paths for Private Files

We've added a fix that properly handles files with a subsite path (e.g. /wp-content/uploads/sites/4/2021/01/unpublished.jpg) with our Private Files feature. We now strip the sites/:id portion of the path before forwarding the request to WordPress.

Checklist

Please make sure the items below have been covered before requesting a review:

• This change works and has been tested locally (or has an appropriate fallback).
• This change works and has been tested on a Go sandbox.
• This change has relevant unit tests (if applicable).
• n/a This change has relevant documentation additions / updates (if applicable).
• I've created a changelog description that aligns with the provided examples.

Steps to Test

See unit tests.

[rinatkhaziev]

Thank you! There's a minor readability suggestion, otherwise looks good!

Needs a re-do

[mjangda]

This needs to be re-thought because of changes in #1975

We can probably handle the stripping to unpublished-files instead since that's the only thing impacted right now.

[mjangda]

Re-thought and should be all good now.

[rinatkhaziev]

@mjangda there's a couple of notes, mostly chores/clerical stuff, but I feel like it's better to address this right away for the sake of our future selves.

[rinatkhaziev]

r1379-stacks